Method of operation and regulation of a vapour compression system

ABSTRACT

A compression refrigeration system includes a compressor ( 1 ), a heat rejector ( 2 ), expansion means ( 3 ) and a heat absorber ( 4 ) connected in a closed circulation circuit that may operate with supercritical high-side pressure.

FIELD OF INVENTION

The present invention relates to compression refrigeration systemincluding a compressor, a heat rejector, an expansion means and a heatabsorber connected in a closed circulation circuit that may operate withsupercritical high-side pressure, using carbon dioxide or a mixturecontaining carbon dioxide as the refrigerant in the system.

DESCRIPTION OF PRIOR ART AND BACKGROUND OF THE INVENTION

Conventional vapour compression systems reject heat by condensation ofthe refrigerant at subcritical pressure given by the saturation pressureat the given temperature. When using a refrigerant with low criticaltemperature, for instance CO₂, the pressure at heat rejection will besupercritical if the temperature of the heat sink is high, for instancehigher than the critical temperature of the refrigerant, in order toobtain efficient operation of the system. The cycle of operation willthen be transcritical, for instance as known from WO 90/07683.Temperature and the high-pressure side will be independent variablescontrary to conventional systems.

WO 94/14016 and WO 97/27437 both describe a simple circuit for realisingsuch a system, in basis comprising a compressor, a heat rejector, anexpansion means and an evaporator connected in a closed circuit. CO₂ isthe preferred refrigerant for both of them.

The system coefficient of performance (COP) for trans-critical vapourcompression systems is strongly affected by the level of the high sidepressure. This is thoroughly explained by Pettersen & Skaugen (1994),who also presents a mathematical expression for the optimum pressure.Based on the fact that the high side pressure is independent fromtemperature, high side pressure can be controlled in order to achieveoptimum energy efficiency. The next step is to determine optimumpressure for given operating conditions.

Several publications and patents are published, which suggests differentstrategies to determine the optimum high side pressure. Inokuty (1922)published a graphic method already in 1922, but it is not applicable forthe present digital controllers.

EP 0 604 417 B1 describe how different signals can be used as steeringparameter for the high side pressure. A suitable signal is the heatrejector refrigerant outlet temperature. The relation between optimumhigh side pressure and the signal temperature is calculated in advanceor measured. Densopatent describes more or less an analogous strategy.Different signals are used as input parameter to a controller, whichbased on the signals regulates the pressure to a predetermined level.

Among others, Liao & Jakobsen (1998) presented an equation, whichcalculates optimum pressure from theoretical input. The equation doesnot take into account practical aspects which may affect the optimumpressure sicnificantly.

Most methods for optimum pressure determination described above, has atheoretical approach. This means that they are not able to compensatefor practical aspects like varying operating conditions, influence ofoil in the system, . . . Optimum pressure will then most probably bedifferent from the calculated one. There is also a risk for a “wind up”and lack of control. The temperature signal gives a feedback to thecontroller, which adjust the pressure with some delay. If conditionschange quit rapidly, the controller will never establish a constantpressure, and cooling capacity will vary.

As explained above, it is a possibility to run tests and measure optimumhigh side pressure relations. But this is time consuming, expensive.Furthermore, it is hard, if not impossible, to cover all operatingconditions. And the measurements has to be performed for all newdesigns.

SUMMARY OF THE INVENTION

A major object of the present invention is to make a simple, efficientsystem that avoids the aforementioned shortcomings and disadvantages.

The invention is characterized by the features as defined in theaccompanying independent claim 1.

Advantageous features of the invention are further defined in theaccompanying independent claims 2-8.

The present invention is based on the system described above, comprisingat least a compressor, a heat rejector, an expansion means and a heatabsorber. It is a new and novel method for optimum operation of such asystem with respect to energy efficiency.

When operating conditions change, the controller in the trans-criticalvapour compression system can perform a perturbation of the high sidepressure and thereby establish a correlation between the pressure andthe energy efficiency, or a suitable parameter reflecting the energyefficiency. A relation between high side pressure and energy efficiencycan then easily be mapped, and optimum pressure determined and useduntil operating conditions change. This is a simple method which willwork for all designs of trans-critical vapour compression systems. Noinitial measurements have to be made, and practical aspects will beaccounted for on site.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further described in the following by way ofexamples only and with reference to the drawings in which,

FIG. 1 illustrates a simple circuit for a vapour compression system.

FIG. 2 shows a temperature entropy diagram for carbon dioxide with anexample of a typical trans-critical cycle.

FIG. 3 shows a schematic diagram showing the principle of optimum highside pressure determination. Temperature approach is used as COPreflecting parameter in the figure.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a conventional vapour compression system comprising acompressor 1, a heat rejector 2, an expansion means 3 and a heatabsorber 4 connected in a closed circulation system.

FIG. 2 shows a trans-critical CO₂ cycle in a temperature entropydiagram. The compression process is indicated as isentropic from state ato b. The refrigerant exit temperature out of the heat rejector c isregarded as constant. Specific work, specific cooling capacity andcoefficient of performance are explained in the figure.

As mentioned above, there is a mathematical expression for high optimumhigh side pressure in a trans-critical vapour compression system. Theexpression is as follows:$( \frac{\partial h_{c}}{\partial p} )_{T} = {- {ɛ( \frac{\partial h_{b}}{\partial p} )}_{s}}$

The optimum pressure is achieved when the marginal increase of capacity(change of h_(c) at constant temperature) equals ε times the marginalincrease in work (change of h_(b) at constant entropy).

Perturbation of the high side pressure, is in principle a practicalapproach to use the equation above. By mapping the energy efficiency, ora parameter which reflects the energy efficiency, as function of highside pressure, it is possible to establish the point where the marginalincrease of capacity equals ε times the marginal increase in work.

Various parameters can be used as reflection for the energy efficiency.

EXAMPLE 1

The temperature difference between refrigerant and heat sink at the coldend of the heat rejector 4, is often denoted as “temperature approach”for a trans-critical cycle. There is a correlation between high sidepressure and the temperature approach. An increase of the high sidepressure will lead to a reduction of temperature approach. The high sidepressure can favourably be increased until a further increase does notlead to a significant reduction of temperature approach. At this point,optimum high side pressure is then in practice established, and thesystem can be operated at optimum conditions, maximizing the system COP.This principle is illustrated in FIG. 3.

A perturbation of the high side pressure will produce a relation asindicated in FIG. 3. When operating conditions change, or for otherreasons, a new perturbation can be made and a new updated relationestablished. In this way, the trans-critical system will always be ableto operate close to optimum conditions.

EXAMPLE 2

Instead of using the temperature approach, it is an option to use thegas cooler outlet temperature as parameter for reflection of energyefficiency.

EXAMPLE 3

By online measurements of system pressures and temperatures, it ispossible to automatically calculate the enthalpies for a trans-criticalcycle at the points 1 to 4 indicated in FIG. 2, if the refrigerantproperties can be provided from property a library. The enthalpies canbe used for calculation of the system coefficient of performance. Aperturbation of the high side pressure will then produce a relationbetween COP and the high side pressure directly.

If COP is used as steering parameter, the optimum high side pressurewill be established directly. If a COP reflecting parameter is used, anexact measure for the “marginal effect” on the parameter has to bequantified. This measure can however easily be estimated. Anotherpossibility is to increase pressure until the parameter reaches apredetermined level.

1-8. (canceled)
 9. A compression refrigeration system including at leasta compressor (1), a heat rejector (2), an expansion means (3) and a heatabsorber (4) connected in a closed circulation circuit that may operatewith supercritical high-side pressure, wherein an online estimation ofcoefficient of performance (COP), or a parameter reflecting the COP, canbe used as a signal for optimum regulation and operation of thecompression refrigeration system.
 10. System according to claim 9,wherein carbon dioxide or a refrigerant mixture containing carbondioxide is applied as the refrigerant in the system.
 11. Systemaccording to claim 9, wherein the regulation system may vary pressure onthe high pressure side in order to map the COP or the COP reflectingparameter as function of pressure for a given operation condition. 12.System according to claim 9, wherein the temperature difference betweenthe refrigerant and heat sink at the cold end (temperature approach) canbe used as a signal for optimum regulation and operation of thecompression refrigeration system.
 13. System according to claim 9,wherein the pressure on the high pressure side of the system can beincreased until the increase has marginal effect on the temperatureapproach.
 14. System according to claim 9, wherein the pressure on thehigh pressure side of the system can be increased until temperatureapproach is equal or lower than a predetermined level.
 15. Systemaccording to claim 14, wherein the predetermined level may vary withvarying operation conditions.
 16. System according to claim 9, whereinthe heat rejector outlet temperature can be used as COP reflectingparameter.